The present invention relates to a GaInP stacked layer structure comprising a GaAs single crystal substrate having stacked on the surface thereof at least a buffer layer, an electron channel layer composed of GaXIn1-XAs (0xe2x89xa6Xxe2x89xa61), a spacer layer composed of GaZIn1-ZP (0xe2x89xa6Zxe2x89xa61), and an electron supply layer composed of GaYIn1-YP (0xe2x89xa6Yxe2x89xa61), and also relates to a field-effect transistor manufactured using the same.
As one kind of field-effect transistors (FET) capable of operating in a millimeter wave region, a GaInP high electron mobility field-effect transistor (simply called TEFGET, MODFET or the like) using a gallium indium phosphide mixed crystal (GaYIn1-YP: 0xe2x89xa6Yxe2x89xa61) is known (see, IEEE Trans. Electron Devices, Vol. 37, No. 10 (1990), pp. 2141-2147). The GaInP MODFET is being used, for example, as a low-noise signal amplification device in the microwave region (see, IEEE Trans. Electron Devices, Vol. 46, No. 1 (1999), pp. 48-54) or a high-frequency transmission device (see, IEEE Trans. Electron Devices, Vol. 44, No. 9 (1997), pp. 1341-1348).
FIG. 4 is a schematic view showing a cross-sectional structure of conventional GaInP TEGFET. For the substrate 90, a semi-insulating gallium arsenide (chemical formula: GaAs) having a {001} crystal face as the main plane is used. On the surface of the substrate 90, a buffer layer 91 composed of a high-resistance III-V compound semiconductor layer is deposited. On the buffer layer 91, an electron channel layer 92 composed of an n-type gallium indium arsenide mixed crystal (GaXIn1-XAs: 0xe2x89xa6Xxe2x89xa61) is deposited. On the electron channel layer 92, a spacer layer 93 is deposited. The spacer layer 93 is generally composed of undoped GaZIn1-ZP (0xe2x89xa6Zxe2x89xa61) (see, IEEE Trans. Electron Devices, Vol. 44 (1997), supra). On the spacer layer 93, an electron supply layer 94 composed of an n-type gallium indium phosphide mixed crystal (GaYIn1-YP: 0xe2x89xa6Yxe2x89xa61) is deposited. The carrier (electron) concentration of the electron supply layer 94 is adjusted by intentionally adding (doping) an n-type impurity with low diffusibility, such as silicon (Si). On the electron supply layer 94, a contact layer 95 composed of n-type GaAs or the like is generally provide for the formation of respective ohmic electrodes of low contact resistance source electrode 96 and drain electrode 97. In the recess structure part between the source and drain electrodes 96 and 97, a Schottky junction-type gate electrode 98 is provided on the exposed surface of the electron supply layer 94, thereby constructing TEGFET 910.
In the region near the junction interface 92b between the electron channel layer 92 and the spacer layer 93 (when a spacer layer 93 is not provided, the electron supply layer 94), electrons fed from the electron supply layer 94 are accumulated as two-dimensional electrons. In general, as the barrier at the junction interface 92b between the electron channel layer 92 and the spacer layer 93 (or electron supply layer 94) is higher, the two-dimensional electrons exerting high mobility can be more efficiently accumulated. As a usual practice, the electron channel layer 92 is composed of GaXIn1-XAs having a constant composition in the thickness direction. The indium composition ratio is mainly about 0.25 (25%) at most (see, Solid-State Electron., 36 (9)(1993), pp. 1235-1237).
However, when the indium composition (=(1-X)) is set almost constant and moreover, to be about 0.25 at most, as in the above-described conventional electron channel layer 92, the attempt to increase the height of the barrier in the vicinity of the junction interface 92b with the spacer layer 93 is limited. Therefore, two-dimensional electrons cannot be efficiently accumulated in the region near the junction interface 92b. As a result, the mobility of two-dimensional electrons cannot be enhanced and this causes a problem that a low-noise GaInP TEGFET using the mobility cannot be obtained.
Under these circumstances, the present invention has been made and an object of the present invention is to provide a GaInP stacked layer structure capable of efficiently accumulating two-dimensional electrons, thereby enhancing mobility of two-dimensional electrons, and being used as a low-noise device using the high mobility. Another object of the present invention includes providing a field-effect transistor manufactured using the GaInP stacked layer structure.
In order to attain the above-described objects, in an embodiment of the present invention, a GaInP stacked layer structure comprises a GaAs single crystal substrate having stacked on the surface thereof at least a buffer layer, an electron channel layer composed of GaXIn1-XAs (0xe2x89xa6Xxe2x89xa61), a spacer layer composed of GaZIn1-ZP (0xe2x89xa6Zxe2x89xa61), and an electron supply layer composed of GaYIn1-YP (0xe2x89xa6Yxe2x89xa61), the electron channel layer contains a compositional gradient region increased in the indium composition ratio (1-X) toward the electron supply layer side.
In a second embodiment of the present invention, in addition to the construction of the above embodiment of the present invention, the compositional gradient region is continuously or discontinuously changed in the indium composition ratio (1-X).
In a third embodiment of the present invention, in addition to the construction of the above embodiments of the present invention, the indium composition ratio (1-X) is from 0.30 to 0.50 at the junction interface in the electron supply layer side.
In a fourth embodiment of the present invention, in addition to the construction of the above embodiments of the present invention, the electron channel layer has a layer thickness of 1 to 5 nanometer.
In a fifth embodiment of the present invention, in addition to the construction of the above embodiments of the present invention, the electron channel layer is composed of n-type GaXIn1-XAs (0xe2x89xa6Xxe2x89xa61) having added thereto boron (symbol of element: B).
In a sixth embodiment of the present invention of claim, in addition to the construction of the above embodiments of the present invention, the spacer layer is composed of GaZIn1-ZP (0xe2x89xa6Zxe2x89xa61) and contains a compositional gradient region reduced in the gallium composition ratio toward the electron supply layer side.
In a seven embodiment of the present invention, in addition to the construction of the above embodiments of the present invention, the spacer layer is not provided.
An eighth embodiment of the present invention relates to a field-effect transistor manufactured using the GaInP staked layer structure of any one of the above embodiments.